Heat the stock to 70°C (158°F). Add the kelp and lemon balm and infuse for 15 minutes. Pass the stock through a sieve lined with muslin (cheesecloth) into a clean saucepan, add the soy sauce and vinegar and check the seasoning. Take a ladle of the seasoned stock and bring to a simmer in a separate small saucepan. Whisk in the rice flour and simmer for 3 minutes. Pass through a fine sieve back into the seasoned stock and gently whisk until very lightly thickened.